Burning fossil fuels such as coal generates a flue gas that contains contaminants including mercury and other trace elements. In addition, the flue gas contains oxides of sulfur and nitrogen (acid gas emissions) and particulates whose release to the environment must be controlled by use of sorbents, scrubbers, filters, precipitators, and other removal technologies. In the example of mercury, mercury is generated in its' elemental form during combustion. Downstream of the boiler, in the ducts and stack of the combustion system, part of the elemental mercury is oxidized. The amount of mercury oxidized depends on the amount of hydrochloric acid (HCl) and other gases present in the flue gas. The amount of mercury in a gas stream varies with different coal, but a typical concentration of mercury in the stream of combustion gas is about 5 parts per billion (ppb). As a result, several pounds of mercury per day may be emitted in some utilities.
Several types of mercury control methods for flue gas have been investigated, including injection of fine sorbent particles into a flue gas duct and passing the flue gas through a sorbent bed. In these types of mercury control methods, the mercury contacts the sorbent and attaches to its surface, where it can be collected along with the fly ash in a baghouse or electrostatic precipitator (ESP). One disadvantage in these prior art systems is that sorbents are used only once and discarded. The sorbents are are not regenerated and reused. These prior art techniques also create solid waste disposal problems, and the spent sorbent may contaminate the collected ash for use in various applications.
Another type of mercury control method uses carbon beds for mercury capture in flue gas. In these types of systems, spent sorbent is typically burned and not regenerated. Some prior art systems employ a recirculating carbon bed, where mercury is removed along with acid gases (as ammonium salts) and the carbon is regenerated at high temperatures where ammonium sulfate is decomposed to SO2 and N2, and mercury desorbs from the sorbent. Attrition of the sorbent results in a significant sorbent cost in this type of control method.
Another type of mercury control method involves injecting manganese oxide sorbent particles in a flue gas stream. For example, U.S. Pat. No. 6,579,507 describes such a method. In this patent, regeneration is claimed by removal of spent oxide particles from the reaction zone and rinsing with dilute aqueous acid. One problem with manganese oxide sorbents is that, while they do oxidize and remove elemental mercury from flue gas, the reaction rates are far slower than those with activated carbons. This means that the MnO2 sorbent can not handle high flow rates of flue gas compared with the injected carbon sorbents. Also, the acid wash of the MnO2 sorbent does not restore the sorbent to its full original activity. In the described MnO2 sorbent system, the purpose of the acid wash treatment is to actually remove the poisoned and spent manganese from the surface of the sorbent as soluble salts, which destroys a portion of the sorbent. The soluble salts contaminate the wash solution, and it is difficult to economically separate and recover the manganese and mercury.
Of interest in designing a mercury control process is to use the sorbent downstream of a particulate control device so the sorbent is not highly diluted with the ash particles. The sorbent could then be more easily regenerated and recycled. The prior art discusses several examples of this type of configuration and sorbent processing.
U.S. Pat. No. 5,607,496 teaches the oxidation of mercury on a metal oxide sorbent bed and subsequent absorption to a sorbent. The sorbent bed follows the particulate removal equipment and thus the gas still contains the SOx and NOx, which react with the metal oxide sorbent to form metal sulfates, that poison the bed. High temperatures are proposed to regenerate the bed. However, mercury is only partially removed from the sorbent at temperatures up to 500 degrees C. Also, the sorbents do not work effectively after regeneration. The cause may be that manganese sulfate formed during the sorption cycle does not completely decompose back to an active manganese oxide form. U.S. Pat. No. 6,383,981 describes a fixed MnO2 or Fe2O3 bed for removal of mercury from a hydrocarbon stream, but no regeneration method is included.
U.S. Pat. No. 6,596,661 describes the regeneration of a plate or honeycomb material composed of transition metal oxides that was used for sorption of mercury in flue gas. The claimed process involves heating the sorbent in a reducing gas stream to remove poisons, followed by impregnation with a poly functional complex-forming reagent containing the catalyst active component to restore mercury capture capacity. The first of these steps can remove the mercury, but it is unclear whether it actually removes the sulfate poison. The second of these steps is rather expensive, because one is actually reconstituting the reagent on the sorbent.
Other types of mercury control methods using porous beds containing a mercury oxidizing reagent on a solid support for removal of mercury from gas streams are described in several patents. For example, U.S. Pat. No. 1,984,164 describes the use of activated carbon or other solid impregnated with a halogen for removing mercury from air. The impregnation method is not specified. No regeneration is claimed.
Examples include peroxomonosulfate (for example, U.S. Pat. No. 4,786,483), triiodide or other mixed halogens (for example, U.S. Pat. Nos. 3,194,629 and 3,662,523), and sulfur (for example, U.S. Pat. Nos. 3,194,629; 4,101,631, 4,708,853, and 6,258,334). In these examples, the reagent material is destroyed either by reaction with the flue gas during sorption or by attempts to regenerate the sorbent. Therefore, these sorbents are not regenerated, except by reimpregnation of the expensive reagent. Amalgamating noble metals (gold, silver) on a suitable support can be regenerated by microwave heating (for example, U.S. Pat. No. 6,136,072), but are expensive and not especially active for sorption in flue gas.